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What is Swamp Fever?

Once your horse is infected, it can have acute or chronic symptoms that can last days to months. Often, symptoms are overlooked or mistaken for another disease until testing is performed. Once the symptoms subside, your horse will remain a lifetime carrier of the virus, and should therefore be quarantined. Because of the highly infectious nature of this virus, it is important to regularly test all the horses in your population often, with a Coggins test.

Swamp fever, or equine infectious anemia (EIA), is caused by an RNA retrovirus. Also called mountain fever, slow fever, equine malarial fever, or Coggins disease, this blood born virus is primarily spread through insect transmission. Symptoms of fever, depression and a general wasting away are characteristic of this virus. There is no recommended treatment and no vaccine to limit the spread of this disease, which can lead to death or euthanization.

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Symptoms of Swamp Fever in Horses

The symptoms of swamp fever can vary, depending on the strength of the infection, and the form your horse may be suffering from. The virus generally incubates for 15 to 45 days before signs of infection appear. They can include:

  • Fever that can last 24 hours, or be recurring
  • Depression
  • Thrombocytopenia, characterized by bruising, bleeding into the tissues, and slow blood clotting seen after an injury
  • Anemia
  • Minute red or purple spots on mucous membranes
  • Pale mucus membranes
  • Jaundice
  • Swelling in the legs, chest and other underbody areas
  • Muscle weakness
  • Emaciation
  • Weight loss
  • Lethargy 
  • Loss of condition 
  • Lack of appetite
  • Increased breathing rate
  • Increased heart rate
  • Irregular heartbeat
  • Blood stained feces
  • Severe decrease in platelet counts
  • Mucosal hemorrhages
  • Death


There are three recognized phases of the EIA virus.

Acute Phase

This initial period lasts about 1 to 3 days, and is characterized by symptoms of fever, thrombocytopenia, and depression. These signs usually don’t last, and are often not noticed. Horses in this phase may die within 2 to 3 weeks.

Chronic Phase 

Following an acute phase, chronic symptoms include recurring episodes spaced out between days to months of high fever, anemia, thrombocytopenia, spotted mucus membranes, swelling, weakness, depression, and weight loss. Veterinarians can see a drop in platelet counts, and such resultant conditions as nonsuppurative hepatitis, glomerulonephritis, meningitis, or encephalitis.

Carrier Phase 

After symptoms have subsided, infected horses remain carriers of the EIA virus for the rest of their lives. Though they have much lower concentrations of the virus in their blood than those horses in the acute and chronic phases, they can still be a source of infection. Stress, strenuous work, and other diseases can trigger a carrier to become acute or chronic.

Causes of Swamp Fever in Horses

The virus that causes swamp fever is a lentivirus, meaning it contains RNA material that produces DNA, which is then incorporated into the genetic material of infected cells. Transmission of the virus is primarily through insect carriers who feed on infected horses, taking the virus to a healthy horse. Although it is considered a blood borne infection, all body fluids and tissues can be infectious as well. EIA can be transmitted by:

  • Blood feeding insects, such as horse flies, stable flies, and deer flies
  • Transmission through contaminated syringes, needles, surgical instruments, and teeth floats. The virus can live up to 96 hours on needles
  • Transmission through blood transfusions
  • Transmission from an infected mare to her foal through the placenta or through her milk
  • Direct or indirect transmission between horses sharing the same living space
  • Transmission through semen
  • Aerosol transmission during close contact

Diagnosis of Swamp Fever in Horses

The diagnosis of swamp fever cannot depend solely on clinical signs, as many of the symptoms can be indicative of another disease. Therefore, virus isolation is often sought to correctly diagnose your horse with equine infectious anemia. This is done through serology testing, most commonly with agar gel immunodiffusion, or a Coggins test, and the ELISAs, or enzyme-linked immunosorbent assays. Because ELISA tests can often give false positives, results from these tests are generally confirmed with a Coggins test. Other tests that can be used are the Western blot test, reverse-transcriptase polymerase chain reaction (RTPCR) assays, and PCR tests.

Serological tests carry the risk of giving false negatives, especially when testing is performed within the first 10 to 14 days after infection. If your horse receives a positive result from a test, you can request another test to verify the result.

Treatment of Swamp Fever in Horses

There is no specific treatment or vaccine available for swamp fever. As such, if your horse tests positive for the virus, your veterinarian will discuss your only options, either euthanasia or a lifetime quarantine of your horse. 

Quarantine is performed within 24 hours after a positive diagnosis is given, and is made on the entire property as well. Infected horses must keep a distance of at least 200 yards from all other horses. The USDA regulates infected horses through the use of a National Uniform Tag code number. This must be administered by a USDA representative in the form of a lip tattoo, hot brand, chemical brand, or freezemark. Interstate movement of infected horses is also regulated, and needs an official permit. All other horses on the property are tested and retested until they are confirmed negative. The property is released from a quarantine when the herd has tested negative for 60 days after the infected horses have been removed.

Asymptomatic mares can give birth to uninfected foals. Foals born to these mares should be isolated from the herd until testing yields negative results.

Recovery of Swamp Fever in Horses

There is no vaccine for the equine infectious anemia virus. There is a risk of death if your horse is suffering an acute or chronic phase of the virus. Carrier horses run a small risk of developing acute or chronic symptoms, but remain vectors of transmission to other horses for the rest of their lives.

Prevention can be difficult, but you can lessen the transmission to the healthy horses on your farm if you have an infected horse through effective quarantine and good hygiene practices. Keep your horses healthy through these strategies:

  • Routinely test all horses every 12 months with a Coggins test
  • Test any horses going out to shows or competitions
  • Isolate and test any new horses coming in to your property
  • Use environmental insect control, such as insecticides and repellants
  • Regularly use good hygiene and maintain disinfection techniques on areas and equipment shared by your horses
  • Use disposable needles and syringes